I get very annoyed when I can't play World of Warcraft because of something like an internet service provider or a local power outage or what have you. Part of the reason is that I raid 25s, which means that 24 other people are expecting me to be there. I know I'm replaceable. That's not really the point. My being replaceable doesn't change the fact that I said I'd be there, and then I wasn't, and it was due to outside factors I couldn't control. One time, I had to leave a raid because during the raid break, on my way down stairs to get a new drink, I tripped and fell and shattered the glass in my hand, cutting up my palm really badly. I felt awful about letting people down that night, and I wasn't even tanking - just DPSing.
For me raiding has always been a social activity - it's something you do with people (potentially as many as 39 other people in the old days) and just like playing on a team sport or in a bowling league, it's something people count on you to show up for. I really hate letting people down. I know it'll happen - I can't make the world care that I'm playing WoW at the specific moment that my entire city is having brown outs.
But it still bugs the heck out of me. I don't care about not being able to play when I have nothing scheduled - I'll go read a book or play something else. But when I've made a commitment, I want to keep it. I hate like fire not being able to.